Patent Document 1 and Patent Document 2 respectively disclose a technology using a slide member which contacts a tooth surface of a worm for stopping rotation of the worm by friction and a technology allowing a worm itself to move in parallel toward a worm wheel and stopping rotation of a worm by friction by pressing the worm against the worm wheel, as a lock mechanism when a seat back is stopped at a predetermined inclination angle. In this structure, a large pressing force is needed for obtaining a necessary lock force.
Between respective teeth of the worm and the worm wheel, a backlash exists for smooth rotating movement, but the backlash is a main cause for rattling or abnormal noise when the seat back is locked.
However, for smooth rotating movement of the worm and the worm wheel, bearings which axially supporting both end portions in an axial direction of the worm have to be strictly positioned to adjust the size of the backlash based on a constant meshing relation. Further, the backlash also varies owing to manufacturing errors of the worm and so on, wear or deformation of teeth due to use, and the like, and thus it is difficult to have a small dispersion per product. Further, even if the worm is pressed against the worm wheel when the seat back is locked, a clearance formed remains between the respective teeth of the worm and the worm wheel along a thrust direction of the worm, and this clearance may cause rattling or abnormal sound when the seat back is locked.
On the other hand, the present applicant has proposed the following mechanisms in Patent Document 3 and Patent Document 4 as a mechanism which can even eliminate the clearance in the thrust direction of the worm when the seat back is locked.
Specifically, Patent Document 3 proposes a reclining adjuster using a worm in which a recessed portion is formed on both end faces, and employing a support structure for the worm in which bearings provided with a recessed portion in faces opposing the worm are fixed to a bracket, and balls are loaded between the worm and the bearings, wherein a tooth part on another end side in an axial direction of the worm is pressed against the worm wheel by a pressing means to lock when the seat back is stopped at a predetermined inclination angle. Further, in order to make a movable range of the worm larger than in the technology of Patent Document 3, Patent Document 4 proposes a mechanism in which a vertically moving movable bearing is provided on the other end side in the axial direction of the worm, and the movable bearing is moved up or down to press the worm against the worm wheel.